When it comes to our weight, there is no need to wallow in the gene pool. Scientists have found that physical activity lessens the link between genes and obesity.

It is well established that bodyweight has a genetic component. With the exception of rare forms of 'monogenic' obesity, which involve one mutated gene, the inherited risk of obesity is shared between many genes, each with a small effect.

Of these, the FTO gene, known as the 'obesity gene', has the greatest effect. Individuals with two copies of the variant carry three kilograms of extra fat and are 1.6 times more likely to be obese, on average. Seventy-five percent of Europeans have at least one copy of the mutated gene and 17 percent have both copies.

Scientists from the Medical Research Council's Epidemiology Unit in Cambridge combined published and unpublished data from 45 studies of adults and nine studies of children and adolescents, a total of nearly 250,000 people. They found that physically active carriers of the variant gene have 27 percent less chance of being obese than those who carry the gene but lead sedentary lifestyles.

'This should convince people who think that their weight is in their genes, and sit back and say they can't do anything about it, that they cando something about it', said the lead author of the study, Dr Ruth Loos, head of the Genetic Aetiology of Obesity Programme at the MRC Epidemiology Unit. 'It's not easy, but you can'.

However the study did not show that being physically active made a difference for children and adolescents carrying the obesity gene. 'That might be just because they are still relatively active, and physical activity at that age does not affect body weight as much as it does in adulthood', Dr Loos explained. 'It may be that later in life, when environment becomes more important, these interactions between genes and environment become more visible as well'.

The researchers set the bar fairly low when they defined physical activity in the study. People were only defined as inactive if they had a sedentary job and engaged in less than one hour a week of exercise, or if their physical activity was in the lowest 20 percent of their study group.

'I think it is important to highlight that you don't have to run a marathon or necessarily join the gym, but walking the dog, cycling to work, taking the stairs... about one hour (of activity) a day, five days a week, will have the effect we saw in our study', said Dr Loos. 'We hope that studies like ours convince people that even when genetically susceptible, a healthy lifestyle will help in the prevention of weight gain'.

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